Interior permanent magnet (IPM) rotor designs using strontium ferrite (ferrite) and neodymium-iron-boron (neo) are known in the art.
In one prior art design, the rotor has a core with long thin slots having neo in each slot. This design does not make use of ferrite. The slots are formed by using a punch press on the rotor core. In order to increase die life, decrease the core weight, and reduce flux leakage, the slots are oversized. The oversized slots allow air spaces around the neo which cause the motor to have high windage noise at high speeds. These motors can have a sinusoidal back electromagnetic flux (EMF) which is desirable.
Another option is to use ferrite in an IPM rotor design. Ferrite is less expensive and can be used to fill large slots. This results in very small air spaces which correspond to a quieter motor. The problem with ferrite is that it does not have a sufficiently high flux density to make an efficient motor.
The combination of neo and ferrite in a single rotor design has been the solution. Large slots near the center of the rotor are filled with ferrite, and smaller slots closer to the edge of the rotor have pieces of neo in them. A motor employing this design is somewhat quieter than a motor using neo alone (i.e. has less windage noise), but generally has a non-sinusoidal back EMF (i.e., it is harmonically rich). Also, the die used in manufacturing this type of rotor has a short lifespan due to the small size of the neo slot.